Due to the increasing energy demand, offshore oil and gas production is moving into deeper waters. Wells for hydrocarbon production can be located in water depths down to 3,000 m or more and a considerable distance from shore. For ensuring an efficient and secure production, processing facilities are being installed at the ocean floor in proximity to such subsea well. Installations on the ocean floor can comprise a range of components, including pumps, compressors and the like which require electric power for operation. Power supply can occur via a subsea power grid installed on the ocean floor, which may for example comprise a subsea transformer 91, a subsea switchgear 92, and a subsea variable speed drive (VSD) 93 for powering subsea loads 94, as shown in FIG. 1.
In order to be capable of powering the subsea installation, voltage is supplied at a high voltage level to subsea transformer 91 and then transformed to a distribution voltage level, which may, for example, be about 36 kV. In such example, switchgear 92 provides 36 kV distribution, and the subsea VSD 93 may provide an output voltage of about 6,000 V. The cables and connectors used between the different components of the subsea installation are rated for the distribution voltage, e.g. for the above-mentioned 36 kV.
When installed subsea, the components of the subsea power grid are exposed to pressures up to or even in excess of 300 bar (at a depth of 3,000 m). To protect the components from the corrosive environment of the surrounding seawater and to deal with the high pressures, pressure resistant enclosures or pressure compensated enclosures can be used.
The components, such as the transformer 91, the switchgear 92 and the variable speed drive 93 are relatively heavy and large sized. Furthermore, they are relatively expensive to produce. In particular for smaller loads, the supply with electric power via components of such subsea power grid might be expensive, and might require these relatively big and heavy modules, such as a subsea variable speed drive 93.
To power such small individual consumers, it is known to provide a single power supply cable, for example from a topside installation, such as an offshore platform or an onshore facility. Although such solution does not require the relatively expensive and heavy subsea components, it requires a subsea cable or an umbilical for each load, which, depending on the length, can also be expensive and difficult to install.
The document EP 2 538 540 A1 discloses a subsea power grid having a power cable, a subsea transformer and a subsea main connection unit for distributing electric power to a number of loads. Each load may comprise a variable speed drive.
The document WO 2009/135730 A1 discloses a power supply device for a variable speed drive arranged at the bottom of the ocean comprising a power converter.
The document WO 2012/164029 A2 discloses electrical architecture for power distribution to subsea equipment comprising at least one variable speed drive.
It is in particular desirable to be capable of powering relatively small individual consumers of a subsea installation located at different positions subsea without the need for such cables or the above mentioned expensive and heavy subsea components.